--- Stuart Rohre <rohre@arlut.utexas.edu> wrote:
>
> There are issues to what makes a ground loop. Any
> dual wire path where the
> ends come together but the dual ground wires are
> somewhat separated forms a
> magnetic loop coupling to and from any source of
> stray AC.
And this also plays a critical role in lightning
protection as well. That same ground loop that causes
you all those pesky problems in a RF field of few
hundred to 1500 watts can be disastrous during a
nearby lightning strike event. A large 200MV @ 200KA
peak lightning pulse packs a lot of RF energy that
will electromagnetically couple into that same ground
loop quite efficiently and will therefore transfer
some serious voltages and currents into your
equipment.
> Wide flat copper strap is
> even better than braid
> in RF low impedance.
The key concept here is the critical importance of
having a low-impedance ground (lightning protection)
system. Most of the lightning strike energy is
distributed from DC to 500KHz or so, however there
still is plenty of problem causing energy well into
the HF spectrum. Ten years ago when I was working on
the lightning protection of a new aircraft design we
needed to address the RF lightning effects well into
the VHF spectrum. We also had to consider the resonant
lengths of all of the components of the aircraft's
structure and systems. Consider that an airplane must
safely transfer that 200MV @ 200KA pulse (actually
there is almost always at least two large pulses) from
one part of the airframe to another. Most often from
nose cone, wing leading edges/tips etc. to the
trailing edges of wings and/or stabilizer surfaces. In
addition to this massive direct lightning effect there
all of the secondary effects primarily consisting of
the electromagnetic coupling of RF energy into the
aircraft?s systems (electrical, fuel, control, etc.)
and other internal structures. If I remember the
statistic correctly the typical commercial airliner is
struck about once per year on average, so the
techniques do work.
> Twisting the heavy conductors
> that are your DC path is
> always a good idea. Running the braid with them or
> using shielded heavy
> power cable is another good idea.
Some examples of ?text book? strategies also used in
aircraft for lightning protection.
> Sadly, few amateur publications adequately separate
> the various purposes of
> grounds, and many hams waste time trying to drive 8
> foot rods in rocky soil,
> or despair of having safe installation and do
> nothing until they are zapped
> by Nature.
This is why I view my station more in the context of
an airplane and what is done to protect them from
lightning damage rather than "grounding". My strategy
is not so much on how to dissipate the lightning
energy but instead to:
1. Transfer the primary (a.k.a. directly conducted)
energy around my gear so it is not conducting the
lightning energy through it. For example by making
sure that my gear is not between any possible sources
and destinations of direct lightning current paths
(antenna leads to AC power systems for example). This
is often the only thing that hams will consider when
trying to protect their gear.
2. Protect the equipment from the secondary effects
such as the electromagnetic coupling of lightning into
the cables and wires. Primarily by making sure that I
don't have any wiring between any of my gear that
looks like a RF transformer's secondary winding
(ground loops fit this description).
3. Make sure that all of my gear rises and falls
together voltage-wise during a lightning event. A
nearby lightning strike will raise the relative local
voltage potential to hundreds if not thousands of
volts. As long as everything rising and falling
together there is no damage.
These 3 points are completely consistent with what
PolyPhaser and others like them describe, it's just a
slightly different way to think about it.
> The safest thing is to unplug all power leads and
> antenna leads from radios
> when your shack is not in use. It is just a
> routine you need to learn, to
> disconnect at end of operations and to reconnect
> before going on air.
...
> The lightning bolt that just traveled 10 miles
across
> the sky will not respect
> the four feet between your computer case and the
> radio antenna feeder, if
> the computer case is grounded well as it should be,
> to the AC power ground.
Disconnecting gear like this does indeed buy you a
large degree of protection, especially for the primary
lightning effects. However it is only effective if the
tower and antennas themselves are properly grounded
with a low-impedance ground system. And that none of
you gear is between any possible lightning
current-flow paths. In reality all that a properly
designed ground system is doing is to simply transfer
the lightning's energy around your gear, it is not
really dissipating anything, which is just like the
airplane transferring the lightning's energy from nose
to tail. And also just like the airplane we want to
make sure that we don't allow any large currents or
voltage differentials be induced into the wiring that
interconnects our gear.
Duane
N9DG
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